As more and more segments of the business environment enter the information age, more and more computers and computing power are required. As businesses move from the old to the new economy their reliance on the processing, transference, and storage of digital information is becoming a more and more critical aspect of their overall business strategy. While in the past, computer crashes were seen as a mere nuisance, the loss of computing power and business data may well devastate a business's ability to survive in today's new economy. As such, the need for reliable, uninterruptible electric power to maintain the operational status of the computing equipment and the integrity of the digital data continues to rise.
To meet these requirements, uninterruptible power supplies (UPS) have been developed that utilize a bank of electric storage batteries and solid state conversion equipment. These UPSs operate in association with the utility line voltage to provide continuous electric power to a business's computer systems in the event of a loss or deviation of power from the utility. The number of batteries contained within an UPS is dependent upon the business's length of time that it needs to operate in the event of a utility power system failure. Likewise, the number of power inverters included in an UPS is dependent upon the overall total system load required to be supplied thereby. In the past, uninterruptible power supplies were only available in discrete sizes. However, with the recognition of the various requirements from the multitude of businesses that require UPSs, the development of modular uninterruptible power supplies has provided a flexibility heretofore unknown. Now, a business's UPS power output may grow as its business needs, without requiring the purchase of additional redundant hardware and control boards as in the past.
In addition to the batteries and power inverters, uninterruptible power supplies also have a requirement for the inclusion of fault bypass circuitry that allows the output loads to be supplied directly by the utility line power once activated. Such bypass circuitry is needed to protect the electronic switches of the power inverter in the event that the output load current draw exceeds the maximum rating of the inverter. Additionally, this circuitry may be activated in the event that a failure within the power inverter is detected by the control circuitry such that the output loads may continue to receive electric power. Under the prior paradigm where uninterruptible power supplies were constructed and sold in a given, fixed, maximum output power capacity, these output circuits were properly sized to handle the rated load of the inverter.
However, as modular UPSs 11 were designed, such as illustrated in FIG. 6, the output bypass circuitry 17 continued to remain a core circuit element within the UPS chassis. As such, it needed to be sized for the maximum configurable power output capacity of the entire modular UPS, regardless of the actual configuration for a particular customer. This ensured that the modular UPS 11 would be able to handle the bypass current requirement for a maximum output power configured UPS, assuming that a customer had installed the maximum number of power inverters 13 supported. Unfortunately, for customers who do not have this maximum power capacity installed (illustrated by empty slots 15a-c), they were still forced to pay the cost for the larger rated circuitry 17. While such additional cost is acceptable to a business that needs its bypass capacity, the additional cost is hard to justify to a customer who may never utilize such high power output.
Further, prior non-modular UPSs had to coordinate operation only between a single inverter and the output bypass circuitry. Therefore, there was no design concern with respect to differential switching of paralleled inverters as is the case with modular UPSs. That is to say, in prior systems the bypass control circuitry needed to only shut-off one inverter before operating the bypass circuit. In many of these UPSs, the output of the inverter was coupled to the loads through a ferroresonant transformer. As such, there was little concern as to the short period between the turning off of the inverter and the turning on of the bypass circuit because the ferroresonant transformer would not reflect this short break in power during the transition therebetween.
Likewise, many modular UPSs also include ferroresonant transformer coupled outputs which also are not concerned with short breaks in power during the transition from the inverter output to the utility line voltage. Since each of these systems could rely on the output power characteristics of the ferroresonant transformer, these prior systems provided the bypass circuitry function by using a simple mechanical relay to bypass the inverter. Unfortunately, these known methods are inapplicable to modular uninterruptible power supplies that utilize transformerless inverters.
Therefore, there exists a need in the art for a new system of providing UPS bypass functionality without requiring that this circuitry be sized for the total output of the possible configuration of the modular UPS chassis, and that is able to operate with transformerless inverters.